Hiya.
Hmmm...I'm thinking it all depends on how many times a character can stand up, then sit down on a chair in the course of a round. I mean, you can't just use common sense here...obviously. Being able to stand/sit without it costing you movement rate...say, 14.3386" per stand/sit (modified by Dex bonus, Acrobatic and/or Athletics check, size, armor, and air humidity)... you're opening up your campaign to all kinds of rules abuse. Best to apply the same level of movement penalty/cost to reaching the top of a cliff/rope climb too, right? *LOL*, I can hear it now "You reach the top of the cliff and stand up, looking down at your foe still climbing after you!" hehehe....can you imagine? Total chaos! (heavens forbid if you are the one climbing up after the bad guy, and the DM asks you to make an Acrobatics/Athletics check to see if you can get up quickly enough else the bad guy at the top gets Initiative and an attack at advantage!... game = instant stop, that's for sure)!
^_^
Paul L. Ming
PS: Sarcasm.